One way of transferring a pattern on to a surface by e-beam lithography is to use a variable shaped beam or VSB to reveal a positive or negative resist coating. For doing so, during a fracturing step, the pattern is to be cut into elementary forms to which a radiation dose is assigned (known as “shots”). Geometry and radiation dose of the shots are closely interrelated because, at the dimensions which are now used (technologies with a critical dimension or “CD” of less than 25 nm), the proximity effects (forward scattering and backward scattering) largely depend on the density of the exposed area.
The patterns to be transferred are quite often of a simple geometric form, such as thin rectangles (lines) or squares (interconnections). These patterns are often named “Manhattan patterns” in the art. Under these circumstances, the geometry of the shots is defined accordingly and is also simple: each pattern is fractured into a union of rectangular or square shots. Nevertheless, for a number of applications (inverse lithography, photonics, metrology calibration, etc.), it may be necessary or advantageous to include in the design patterns which are not simple forms of the type previously described, but which may be circles or of an indeterminate, possibly curvilinear, form (further referred to as free-form).
It is known in the art to apply geometry and/or dose corrections to the shots to obtain an insulated or developed set of patterns which is the closest possible to the target design. For instance, a combined optimization of the shot size and their dose is disclosed by European Patent application published under n°EP2559054. In the case of free form patterns, paving by shots with a circular or quasi-circular imprint of varying size is disclosed by PCT application filed under n°PCT/EP2013/053883 which is assigned to the applicant of this application.
It is also known to apply corrections before fracturing. For instance, edge based corrections may be calculated by comparing a simulated contour and a target contour and implemented by applying iteratively edge displacement vectors to the simulated contour until a matching criteria is reached. Such a technique is disclosed by the PCT application published under n°WO2014/177776 which is also assigned to the applicant of this application. For Manhattan type design edge based corrections improve on shot based corrections because they create far less artefacts, like overlaps, holes, lonely thin shots, edges which cannot be moved, etc.). But in the case of free-form patterns, edge based corrections are far less efficient because, notably, they allow less control of the impact of the corrections on the simulated contour.
It would be advantageous to find a novel technique which brings the benefits of edge based corrections without its drawbacks.